Wireless communication systems including electronic identification devices, such as radio frequency identification device (RFID) cards, are known in the art. RFID cards frequently include a unique serial number permanently and unalterably burned into an integrated circuit contained within the card. The integrated circuit typically has sufficient memory capacity for data (e.g., electronically or optically stored) such as a card issuer identification (ID) number, user information (name, account number, signature image, etc.), the private key of a public-private key pair, a digital signature, and a personal identification number (PIN).
Wireless communication systems employ an RF transponder device to which an interrogator transmits an interrogation signal. If within range, the device receives the signal then generates and transmits a responsive signal. In the case of an active device having an on-board power source, close proximity to an interrogator or reader is not required.
With reference to FIG. 1, an RFID card 100 of the prior art includes a plastic housing card 103 and a radio frequency communication device 105. The radio frequency communication device 105 includes an integrated circuit 107, at least one antenna 109 connected to the integrated circuit 107 for radio frequency transmission and reception of data to and from the integrated circuit 107, and a power source 111 to supply power to the integrated circuit 107. The integrated circuit 107 includes receiver and transmitter sections (not shown). The power source 111 is a battery or other suitable power source. Alternatively, the power source 111 may not be physically present within the plastic housing card 103. In such cases, the radio frequency communication device 105 derives power inductively when in proximity to an interrogator unit, described below.
Various configurations are possible for the antenna 109. In the RFID card 100 of the prior art, the antenna 109 is shared by the receiver and transmitter sections of the integrated circuit 107. The antenna 109 is frequently formed by conductive epoxy screened onto or within laminations of the plastic housing card 103. The antenna 109 typically comprises a folded dipole antenna defining a continuous conductive path, or loop, of microstrip. A microstrip is a thin, flat electrical conductor separated from a ground plane by a layer of insulation or an air gap. Alternatively, the antenna 109 can be constructed as a continuous loop antenna.
In FIG. 2, a radio frequency communication system 200 of the prior art includes the RFID card 100 and a radio frequency interrogator unit 201. The RFID card 100 transmits and receives radio frequency communications to and from the interrogator unit 201. The interrogator unit 201 is ordinarily a large, stationary unit and includes an antenna 203 as well as dedicated transmitting and receiving circuitry, similar to that implemented on the integrated circuit 107 of the RFID card 100. The radio frequency communication system 200 further includes a host computer 205 in communication with the interrogator unit 201. The host computer 205 acts as a master in a master-slave relationship with the interrogator unit 201. The host computer 205 typically includes an applications program for controlling the interrogator unit 201 and interpreting responses. Further, the host computer 205 frequently contains a library of radio frequency identification device applications or functions. The functions affect radio frequency communication between the interrogator unit 201 and the RFID card 100.
Generally, the antenna 203 transmits an interrogation signal 207. The RFID card 100 receives the incoming interrogation signal via the antenna 109. Upon receiving the signal 207, the RFID card 100 responds by generating and transmitting a responsive signal 209. The responsive signal 209 is encoded with information that uniquely identifies or labels the RFID card 100 that is transmitting, thereby identifying any object or person with which the RFID card 100 is associated.
However, a plurality of RFID cards (not shown) could simultaneously be within the interrogative field of the interrogator unit 201 (i.e., within communications range of the interrogator unit 201). Additionally, one or more RFID cards may simply be passing by in range of the interrogator unit 201. As soon as any RFID card comes within range of the interrogator unit 201, communications may be initiated, perhaps surreptitiously. As distance ranges for reading of RFID cards are ever increasing, the problem with wrongful card reading becomes increasingly problematic. For example, a person carrying an RFID card in a wallet or purse may unknowingly come within range of an interrogator and communications may be initiated unknowingly. As communications is initiated, an unscrupulous malefactor may intercept data contained on the RFID card. Therefore, what is needed is a simple and effective means of preventing unknowing and unwanted communications between an RFID card and an interrogator unit.